I think the big reason football is considered a 'fast' sport is two-fold. One: The total number of plays in a games is significantly lower than the total number of plays (ie pitches) in a baseball game. Two: While it's true that in both games, something big can happen on any given play/pitch, the number of non-event pitches is significantly higher in proportion to non-event downs.
While I don't want this done, to make baseball a 'fast' game, you'd need to change the number of balls for a walk down to two and change it so you get three strikes period. No fouling pitches off. You either put the ball in play or you are out. With a cap of five pitches per at bat, period and more first pitch swinging, it would be a MUCH faster game, even with the same delay between pitches.
Again, I ain't saying I want that. Just saying that's why football is 'fast' even though it's a lot of 5 seconds of action, 40 seconds to reset.